This is an interesting question, because actually, in the second meeting that Macbeth has with the witches, they don't really "say" that much. Rather, in response to his demands for answers, they conjure up a series of apparitions that seem to taunt and confuse Macbeth even further with false hopes and equivocal meanings. The lines that they do deliver point towards the double meaning of the visions that they show Macbeth, and the way that they know their meaning will hurt him. Consider the following lines, for example:
Show his eyes and grieve his heart.
Come like shadows; so depart.
They know that the visions Macbeth is so insistent on seeing will "grieve his heart," and of course this is part of their manipulation of him and their use of him for their evil and nefarious purposes. The First Witch finally taunts Macbeth yet again before the witches disappear by saying:
Come, sisters, cheer we up his sprites
And show the best of our delights.
I'll charm the air to give a sound
While you perform your antic round,
That this great king may kindly say
Our duties did his welcome pay.
Certainly we should not be blind to the irony in addressing Macbeth as "this great king." Whilst he certainly is king, the witches know what Macbeth has done to gain the crown, and are also aware that his rule will be but temporary. Thus we can say that there is a certain amount of sarcasm in these lines.